Microwave tube



Sept 8, 1959 R. H. BARTRAM 2,903,620

MICROWAVE TUBE Filed sept. 6, 1957 INVENTOR. RALPH h'. BART/PAM MgrATTORNEY United States Patent O M MICROWAVE TUBE Ralph H. Bartram, KewGardens, N.Y., assignor, by mesne assignments, to Sylvania ElectricProducts Inc., Wilmington, Del., a corporation of Delaware ApplicationSeptember 6, 1957, Serial No. 682,458

3 Claims. (Cl. 315-18) My invention is directed toward strophotronoscillators.

A strophotron oscillator is a multitransit electron tube adaptable foruse in the VHF and UHF` frequency ranges and is described, for example,in an article entitled A New Electron Tube: The strophotron by HannesAlfven and Dag Romell, published in the Proceedings of the I.R.E., page1239, vol. 42, No. 8, August 1954.

In my copending application Serial No. 680,761, led August 28, 1957 Idisclosed a new and improved strophotron provided with an elongatedaccelerator having a uniform cross section deiining an upper branch ofan hyperbola. This strophotron further includes rst and Second separatedplane reflectors so positioned as to coincide with correspondingportions of the rst and second asymptotes of the upper hyperbola branch,the reflectors and the accelerator extending in the same direction.

The accelerator is maintained at a high positive potential relative tothe reectors, thus establishing an electric field therebetween. Thisield, due to the geometry of the accelerator and reilectors, creates anhyperbolic electric potential distribution within the entire regionbounded by the accelerator and the reflectors. Stated differently, thepotential V of any point within this region is proportional to thequantity (y2-x2), wherein y is the vertical separation between thispoint and the intersection of the lirst and second asymptotes and x isthe horizontal separation between this point and this intersection.

A uniform magnetic field is established Within this region, the magneticfield vector pointing in a direction perpendicular to a plane ofsymmetry extending equidistantly between the reflectors and passingthrough the central point of the upper hyperbola branch.

A cathode is mounted in one of the reflectors adjacent one end of theaccelerator, and an anode or collector maintained at a high positivepotential relative to the reilectors is positioned adjacent the otherend of the accelerator. A load, which can be for example a resistor or aresonant circuit tuned to the oscillator frequency, is coupled betweenthe two reectors.

Under these conditions, electrons emitted from the cathode will migratetoward the collector along a curved path and will exhibit thecharacteristic strophotron behavior; i.e. the projection of this pathonto the plane of symmetry resembles a trochoid, while the projection ofthis path onto a second plane, perpendicular to the plane of symmetryand extending in the direction of the reflectors, resembles a dampedsinusoid having an axis defined by the intersection of the two planes.

However, in common with the known strophotrons, the above describedstrophotron suffers from certain common disadvantages. For example, fora xed cathode position, the magnetic field intensity required increasesas the oscillation frequency increases, and eventually the requiredintensity becomes prohibitively high.

The required magnetic field intensity can be reduced by increasing thevertical separation between the cathode and the central point of thehyperbolically shaped por- 2,903,620 Patented Sept. 8, 1959 tion of theaccelerator. However, as this separation is increased, the directpotential at the point at which the emitted electrons intersect theplane of symmetry will decrease and eventually will be substantiallyequal to the direct reflector potential. The power output of thestrophotron for a fixed tube geometry and fixed values of appliedpotentials, varies directly with the difference in direct potentialbetween the reiiectors and this point of intersection. Hence, as thevertical separation between the cathode and the central point of thehyperbola portion decreases, the power output of a strophotron alsodecreases.

I have invented a new type of strophotron which overcomes thesediiculties.

Accordingly it is an object of the present invention to provide a newand improved strophotron of the character described.

Another object is to improve strophotron operation by reducing themagnetic eld intensity required for operation at any given frequencywithin the strophotron frequency range.

Still another object is to increase the power output of a strophotronhaving a fixed tube geometry and iixed values of applied potentials.

These and other objects of my invention will either be explained or willbecome apparent hereinafter.

In accordance with the principles of my invention, I provide rst andsecond separate, electrically conductive, elongated accelerators, eachof which has a uniform cross section defining a branch of an hyperbola.The accelerators are placed parallel to each other and are so orientedas to respectively form upper and lower branches of an hyperbola. Hence,the two accelerators are disposed about a first plane of symmetry whichextends in the same direction as the accelerators and which passesthrough the central points of both hyperbola branches.

I further provide rst and second separate, electrically conductive,elongated reflectors extending in the same direction as theaccelerators, each reilector having a uniform cross section defining abranch of an hyperbola. The two reflectors are symmetrically disposedabout opposite sides of the iirst plane of symmetry. Hence, the tworeectors are disposed about a second plane of symmetry perpendicular tothe irst plane of symmetry and passing through the central points ofboth reflectors. Each reflector together with its adjacent acceleratordefines a common hyperbolic asymptote therebetween.

The accelerators are maintained at a high positive potential relative tothe reflectors, thus establishing an electric field therebetween. Thisiield, due to the geometry of the accelerators and the reiiectors,establishes an hyperbolic electric potential distribution within theelectron interaction region bounded by the reflectors and accelerators.More particularly, the potential V at any point within this region isproportional to the quantity (y2-x2) wherein y is the perpendicularseparation between this point and the second plane of symmetry and x isthe perpendicular separation between this point and the first plane ofsymmetry.

A uniform magnetic eld is established within the electron interactionregion, the magnetic eld vector pointing in a direction perpendicular tothe first plane of symmetry.

A cathode is mounted in one of the reectors at a point intermediate thecentral point of the said one reflector and one of the accelerators andadjacent one end of both accelerators. An anode or collector maintainedat a high positive potential relative to the reflectors is positionedadjacent the other end of both accelerators. A load is coupled betweenthe two reflectors.

Under these conditions, electrons emitted from the cathode will migratetoward the collector along a curve confined between said one acceleratorand the second plane of symmetry and will again exhibit thecharacteristic strophotron behavior; i.e. the projection of this pathonto the rst plane of symmetry resembles a trochoid, while theprojection of this path onto the second plane of symmetry resembles adamped sinusoid having an axis defined by the intersection of the twoplanes.

By virtue of this arrangement, for a given power output, the requiredmagnetic field intensity can be substantially reduced over that hithertorequired. Converse- 1y, for a given field intensity, the power outputcan be substantially increased over that hitherto obtainable.

An illustrative embodiment of my invention will now be described withreference to the accompanying drawings, wherein Fig. l is an isometricview of a strophotron in accordance with my invention;

Fig. 2 is a graph of the electric potential distribution between theaccelerators and the reflectors shown in Fig. 1; and

Figs. 3a and 3b are graphs of the motion of favorably phased electronsin the interaction region between the accelerators and the reflectors.

Referring now to Fig. l, enclosed in an evacuated tube envelope (notshown) is a pair of first and second, vertically displaced, electricallyconductive, elongated accelerators and 12. Each accelerator has auniform cross section defining a branch of yan hyperbola. The firstaccelerator 10 forms an upper hyperbola branch having a central point14, while the second accelerator 12 forms a lower hyperbola branchhaving a central point 16. Thus, the two accelerators are disposed upona first plane of symmetry which extends in the same direction as theaccelerators and which passes through central points |14 and 16.

Further provided are first and second horizontally displaced,electrically conductive, elongated reflectors 18 and 20 extending in thesame direction as accelerators 10 and 12. Each reflector has a uniformcross section defining a branch of an hyperbola. The two reflectors aresymmetrically disposed about opposite sides of the first plane ofsymmetry so as to define left and right hand hyperbola branches; theleft hand branch having a central point 22, the right hand branch havinga central point 24. Hence, the two reflectors are disposed about asecond plane of symmetry perpendicular to the first plane of symmetryand extending through the central points 22 and 24.

A uniform magnetic field is established within the region bounded by theaccelerators and reflectors, the magnetic field vector pointing in adirection perpendicular to the first plane. (Means for establishing thisfield are conventional and are not shown here.) Each reflector togetherwith its adjacent accelerator defines a common hyperbola asymptotetherebetween; consequently, there are four asymptotes 26, 28, 30 and 32.

The two accelerators 10 and 12 are coupled to a point of positive directpotential -l-V1. The two reflectors are coupled to a second point ofnegative potential -V2. As a result, the electric field establishedbetween the accelerators and reflectors establishes an hyperbolicelectric potential distribution within the electron interaction region.(Note that the accelerators and the reflectors are extended sufficientlyto substantially eliminate fringe electric fields.)

A cathode 34 is mounted in reflector 18 at a point intermediate thecentral point 22 of this reflector and accelerator 10, the cathode beingadjacent one end of both electrodes. An anode or collector 36 maintainedfor convenience at the same potential as the accelerators 10 and 12, ispositioned adjacent the other end of both accelerators. A load 38 whichcan be purely resistive, but, in this example, is a resonant circuittuned to the oscillation frequency, is coupled between reflectors 18 and20.

Electrons are emitted from the cathode at an extremely low velocity andenter the interaction region. Many of these electrons then drift towardthe collector along a curved path and exhibit the characteristicstrophotron behavior.

More particularly, the projection of this path onto the first plane ofsymmetry resembles a trochoid as shown in Fig. 3a. Further, theprojection of this path onto the second plane of symmetry resembles adamped sinusoid as shown in Fig. 3b. The sinusoidal frequency is theoscillation frequency and is primarily determined by the electric fieldestablished between the accelerators and the reflectors; the trochoidfrequency is determined by both the magnetic and electric fields and isindependent of the oscillation frequency. (The magnetic eld intensity isadjusted to a value at which electrons are prevented from impinging onand being collected by the accelerator.)

The hyperbolic electric field distribution between the accelerators andthe reflectors is plotted graphically in Fig. 2. As will be seen fromFig. 2, the electric potential V for any point P between theaccelerators and the rellectors is proportional to the quantity (y2-x2),where y is the vertical separation between point P and a line 50extending between central points 20 and 22, and x is the horizontalseparation between point P and a line 52 extending between centralpoints 14 and 16. The equi-potential surfaces 54 are families of rightliyperbolas with the lines y=ix as asymptotes 26, 28, 30, 32. Note thatsince the accelerators define surfaces of |V1 potential and thereflectors define surfaces of -V2, the asymptotes 26, 28, 30 and 32define lines of an intermediate potential V3.

The oscillation frequency, as indicated previously, is determined by theelectric field. Further, the electrons, in moving toward and away fromthe reflectors, induce an alternating voltage of oscillator frequencyacross the load, an-d this voltage acts on the electrons to produce thedamping action. More specifically, when an electron leaves the cathodeand enters the interaction region at such times as to be in phase withthe induced voltage (such an electron is termed a favorably phasedelectron), it will oscillate back and forth between the planes in themanner indicated. The electrons will enter the interaction region atsuch times as to be out of phase with the induced voltage impinge on areflector on the first or second pass, and are thus removed from theinteraction region. Hence, only the favorably phased electrons remain inthe interaction region for an appreciable period, and the oscillatorvoltage appearing across the load is not substantially affected by outof phase electrons.

The amplitude of the sinusoidal motion of any electron is ydetermined bythe vertical displacement between this electron and the central point ofthe appropriate hyperbola branch, the ampltude decreasing as thedisplacement increases.

However, by virtue of the hyperbolic electric field distribution theoscillation frequency remains constant despite changes of verticaldisplacement.

It will be seen from a study of Figs. l and 2 that the potential at thepoint at which the electrons emitted from cathode 34 intersect plane Awill vary between the limits of +V1 and V3 depending upon the separationbetween cathode 34 and the central point 22 of accelerator 10.

In the prior art of which I am aware the reflectors are maintained atground potential; hence, in this situation, as the cathode is moved awayfrom the accelerator, the potential of the point of intersection ofplane A approaches V3 and the power output (which varies directly withthe difference in potential between this point and the reflectors)approaches zero. In contradistinction, in my invention the reflectorsare maintained at a potential of -V2, and as the cathode is moved awayfrom the accelerator, the power output decreases at a much slower rateand ultimately attains a reasonably large finite value.

Further, for a given power output, the conventional cathode-acceleratorseparation must be substantially smaller than the separation required inmy invention, and hence the required magnetic field intensity in myinvention under these conditions is substantially less than thatrequired by the prior art.

While I have shown and pointed out my invention as applied above, itwill be apparent to those skilled in the art that many modifications canbe made within the scope and sphere of my invention as defined in theclaims which follow.

What is claimed is:

1. In a strophotron, first `and second, vertically separated,electrically conductive, elongated accelerators extending in the samedirection, each accelerator having a uniform cross section defining abranch of an hyperbola, said accelerators having positions at which, incross section, said first and second accelerators respectively formupper and lower hyperbola branches; first and second horizontallyseparated, electrically conductive, elongated reflectors extending insaid direction, each reflector having a uniform cross section defining abranch of an hyperbola, sai-d reflectors having positions at which, incross section, said first and second reflectors respectively form lefthand and right hand hyperbola branches, any reflector together with anyadjacent accelerator defining a common hyperbolic asymptotetherebetween; first means coupled to said accelerators to establish afirst direct potential thereon; second means coupled to said reflectorsto establish a second direct potential thereon, said first potentialbeing more positive than said second potential; means to establish -atime invariant magnetic field about said reectors and said accelerators,the magnetic field vector pointing perpendicular to a plane extendingbetween the central points of the upper and lower hyperbolic branches insaid directiong'said first reflector being provided with a slot adjacentone end thereof; -a cathode mounted within said slot; and a collectorpositioned adjacent said accelerators and said reflectors at the otherend of said first reflector, said collector being coupled to said rstmeans.

2. In combination, first, second, third and fourth separated, elongated,electrically conductive members eX- tending in the same direction, eachmember having a uniform cross section defining a branch of an hyperbola,said first and second members being designated as accelerators andhaving positions at which the central points of their respectivehyperbola sections face each other thereby defining a first plane ofsymmetry extending between said accelerator central points in saiddirection, said third and fourth members being designated as refiectorsand being symmetrically disposed about opposite sides of said firstplane, said reflectors having positions at which their respectivehyperbola sections face each other thereby defining a second plane ofsymmetry extending between said reflector central points in saiddirection, said first and second planes being mutually perpendicular,means to establish a time invariant magnetic field having its fieldvector pointing perpendicular to said first plane, said third memberbeing provided with a slot adjacent one end thereof; a cathode mountedwithin said slot; and a collector positioned adjacent said first,second, third and fourth members at the other end of said third member.

3. In combination, first, second, third and fourth separated, elongated,electrically conductive members extending in the same direction, eachmember having a uniform cross section defining a branch of an hyperbola,said members having positions at which said first, second, third andfourth members in cross section respectively define upper, lower, lefthand and right hand hyperbola branches, any two adjacent membersdefining a common hyperbolic asymptote therebetween, said first and'second member being electrically interconnected to a first point ofdirect potential, said third and fourth members being electricallyinterconnected to a second point of direct potential, said first pointbeing more positive than said second point, means to establish a timeinvariant magnetic field about all of said members, the magnetic fieldvector pointing parallel to a line joining the central points of saidleft hand and right hand hyperbola branches, said third member beingprovided with a slot adjacent one end thereof; a cathode mounted withinsaid slot; and a collector positioned adjacent said first, second, thirdand fourth members at the other end of said third member, said collectorbeing coupled to a third point of direct potential, said third pointbeing at least as positive as said first point.

References Cited in the le of this patent UNITED STATES PATENTS2,124,270 Broadway July 19, 1938 2,293,567 Skellett Aug. 18, 19422,414,121 Pierce Jan. 14, 1947 2,520,813 Rudenberg Aug. 29, 19502,536,150 Backmark et al J an. 2, 1951 2,834,908 Kompfner May 13, 1958FOREIGN PATENTS 729,930 Great Britain May 11, 1955

